Plutonium separation method



2,819,143 Patented Jan. 7, 1958 PLUTONIUM SEPARATION WTHOD John L. Dreher, El Cerrito, and Stanley G. Thompson, Richmond, Cam assignors to the United States of America as represented by the United States Atomic Energy Commission No Drag. Application .i'anuary 12, 1948 Serial No. 1,896

Claims. (Cl. 23-145) This invention is concerned with an improved method of separating plutonium from contaminating elements normally associated with plutonium in neutron-irradiated uranium.

The word plutonium, as hereinafter used in the specification and claims, refers to the element of atomic number 94 and to the compounds thereof, unless the context indicates clearly that plutonium is referred to in its metallic state.

Natural uranium is composed of three isotopes; namely, U U and U the latter being in excess of 99% of the whole. When U is subjected to the action of slow or thermal neutrons, a fourth isotope, U is produced having a half-life of twenty-three minutes and undergoing beta decay to Np which in turn decays with a half-life of two and three-tenths days to yield plutonium. In addition to the formation of 94 there are simultaneously produced other elements of lower atomic weight known as fission fragments. These fission fragments are composed of elements having atomic numbers from about 32 to 64. The elements of this group, as originally produced, are considerably overmassed and undercharged and hence are highly unstable. By emission of beta particles accompanied by gamma radiation, these elements transform themselves into isotopes of these various elements having longer half-lives. The resulting materials are commonly known as fission products.

Various radioactive fission products have half-lives ranging from a fraction of a second to thousands of years. Those having very short half-lives may be substantially eliminated by aging the material for a reasonable period before handling. Those with very long half-lives do not have sufficiently intense radiation to endanger personnel protected by moderate shielding. On the other hand, the fission products having half-lives ranging from a few days to a few years have dangerously intense radiations which cannot be eliminated by aging for practical storage periods. These products are chiefly radioactive isotopes of Sr, Y, Zr, Cb, Te, I, Cs, Ba, La, Ce, and Pr.

It may be readily seen that plutonium, as produced by the process generally set forth above, is contaminated with considerable quantities of uranium and fission products. In fact, the plutonium constitutes only a very minor portion of the irradiated mass, i. e., less than 1% thereof. In view of such low concentration of plutonium in the irradiated metal and the highly radioactive charactor of the fission products present, it becomes apparent that the procedure employed to recover that element must be highly eflicient in order to be at all practicable.

There have been devised a number of procedures for the removal and concentration of plutonium from extremely dilute solutions thereof. In general, such methods involve the formation of various insoluble compounds in said dilute solutions, capable of carrying plutonium in one of its valance states. The carrier precipitate and plutonium thus obtained are then dissolved and the plutonium converted to another valence state, in which state it is soluble in the presence of said carrier. The carrier is then re-precipitated from the new solution, thus removing the fission products present in this solution, but leaving the soluble plutonium in solution. Thereafter, the plutonium may be separated from the solution directly as a plutonium compound or the plutonium may be converted to the valence state in which it is insoluble with the aforementioned carrier and removed from the solution with that carrier. This carrier precipitate may again be dissolved and the plutonium may be purified further, if considered necessary or desirable, by repeating the above cycle. A carrier precipitate which carries plutonium is usually refered to as a product precipitate, while a carrier precipitate which removes elements other than plutonium, leaving the plutonium in the solution, is usually referred to as a by-product precipitate.

The precipitation method of separation is usually divided into four steps. The steps are: Extraction, in which the plutonium is separated from the uranium and most of the fission products; Decontamination, in which the plutonium is separated from the remaining fission products; Concentration, in which the ratio of carrier precipitate of plutonium containing solution to plutonium is greatly reduced; and Isolation, in which the final plutonium compound is precipitated directly from solution.

One of the most important of the plutonium separation methods based upon precipitation of plutonium compounds is the fluoride method. This separation procedure depends upon the fact that plutonium in the hexavalent state forms a water-soluble fluoride but plutonium in the trivalent and tetravalent states forms water-insoluble fluorides. The water-insoluble tetravalent plutonium fluoride is quantitatively carried from an aqueous solution by a carrier precipitate consisting of a trifluoride of a rare earth metal of the cerium sub-group consisting of elements 57 to 62, inclusive, such as lanthanum or cerium trifluoride.

While the wet fluoride process is highly efficient in the separation of plutonium from contaminants normally associated with plutonium in neutron-irradiated uranium, particularly the radioactive fission products, there are certain disadvantages and inconveniences in the use of this process. The insoluble fluoride formed with lanthanum or cerium and tetravalent plutonium is quite insoluble in acid solutions so that it requires a considerable quantity of a strong concentrated inorganic acid to dissolve the mixed plutonium and rare earths fluorides. This acid solution is then greatly diluted before the plutonium is oxidized and a by-product carrier precipitate is separated from the solution. Because of thi insolubility of the fluorides the decontamination cycle of the fluoride process leaves the plutonium in a no more concentrated condition in the solution at the end of the cycle than at the beginning. An additional step is therefore required to concentrate the plutonium contained in the solution.

It is an object of this invention to provide a convenient and efficient method of recovering plutonium from impurities commonly associated therewith in a neutron-irradiated uranium mass.

An additional object of this invention is to provide an improved fluoride decontamination cycle whereby the concentration of the plutonium in the plutonium-containing solution is greatly increased.

Additional objects of this invention will be apparent from the following detailed description.

Broadly, this invention comprises first introducing a plutonium carrier precipitate into an aqueous acidic solution containing ceric ion. The carrier consists of the trifluoride of a member of the cerium sub-group of rare earths and may contain, in addition to triand tetravalent plutonium, certain radio active fission products which are fluoride-insoluble. The ceric ion, which is present in the aqueous acidic solution in at least equimolar concentration with the carrier cation, acts not only to dissolve the fluoride precipitate, but also to oxidize the plutonium to the hexavalent state. Following the oxidation of the plutonium a by-product carrier precipitate is formed in the plutonium-containing solution by introducing a soluble alkali metal compound and a soluble fluoride compound into the solution. The by-product precipitate thus formed is then separated from the solution, leaving the plutonium in solution in the hexavalent state, substantially free from radioactive fission products. The plutonium is then reduced and separated from the solution with a much smaller carrier precipitate of a trifluoride of a cerium sub-group rare earth metal.

While the theory underlying the operation of this invention has not been definitely determined and the inventors do not wish to be bound by any theory advanced, it is believed that the trifluoride of a rare earth metal of the cerium sub-group is dissolved by the action of the ceric ion in forming a complex ion with the fluoride ions (believed to be C6135). The action of the ceric ion in oxidizing plutonium is Well known. The ceric fluoride complex forms a very insoluble precipitate with an alkali metal, such as potassium or sodium, and thus the addition of a soluble alkali metal compound and a soluble fluoride compound, preferably hydrofluoric acid, causes quantitative precipitation not only of all excess ceric ions and ceric fluoride complex ions as the alkali metal ceric fluoride, but also the lanthanum or cerous ions used in the original carrier as a cerous or lanthanum fluoride. This precipitate acts as an effective by-product carrier for fluoride-insoluble contaminants, removing substantially all of the radioactive contaminants carried with the original fluoride carrier precipitate.

The numerous advantages of this invention will be readily apparent to those skilled in the art. The double action of the ceric ion in dissolving the plutonium carrier precipitate and oxidizing plutonium, results in a considerable saving of time and a reduced number of operations, and the small volume of acid required to dissolve the plutonium carrier causes a great concentration of the plutonium over that obtained by previous methods.

In its preferred embodiment the process of this invention is used to replace the decontamination and concentration steps of the bismuth phosphate-lanthanum fluoride plutonium separation process, so that the extraction and isolation steps of the bismuth phosphate-lanthanum fluoride process, coupled with the process of this invention, form a simplified and improved method of separating plutonium from contaminants normally associated with it in neutron-irradiated uranium. In the bismuth phosphatedanthanum fluoride process the neutron-irradiated uranium is dissolved in nitric acid to form a uranyl nitrate hexahydrate solution. This solution, which contains hexavalent uranium, tetravalent plutonium and radioactivc ion products, is treated with a bismuth phoswhich removes plutonium and phosphateon products, leaving the phosphate-soluble .-...sion products and the bulk of the uranium in the solution. This bismuth phosphate plutonium carrier is dissolved aud a lanthanum fluoride plutonium carrier is then pre ated from this solution. This carrier precipitate car M t e plutonium and a smaller amount of fission products. y the process of this invention, this lanthanum fluori atonium carrier is introduced into an inorganic acid 5 on containing ceric ion. The acid in the solution, nitric or sulfuric preferably, is present in about a to 2 N concentration. The amount of ceric ion in the solution should not be less than the amount of carrier 7 Le cation introduced into the solution, and if the carr .r is lanthanum fluoride, it is preferable to have a ratio of ceric to lanthanum ion of about 2 to 1. If the carrier precipitate is cerous fluoride, the l-to-l ratio of ceric to cerous ion is quite satisfactory. The ceric ion.

may be added to the dilute acid solution by dissolving a soluble ceric salt, such as Ce(HSO in the solution.

The proportion of ceric ion to carrier cation given in the paragraph above is usually ample not only to dissolve the carrier precipitate, but also to oxidize the plutonium, introduced into the solution with the carrier precipitate, to the hexavalent state. The carrier precipitate may be dissolved at room temperature or at higher temperatures, and the plutonium may be oxidized at room or higher temperatures. It has been found that the rate of oxidation is som what increased by digesting the solution at elevated temperatures, for example fill- C. for from one-half to one and one-half hours. This is not at all critical, however, and both dissolution and oxidation may be carried out at room temperatures by allowing more time, or increasing the ratio of ceric ion to carrier cation. The following table shows the oxidation conditions for several specific examples. The plutonium Was contained in the solution in tracer quantity in these experiments.

TABLE Oxidation of Pu. by Co in ceric cycle of the wet fluoride process Oxidation Conditions Percent H H Pu oxi- Q- ma e (Je Time, Temp, HNOs, H2804, dized g 0e+3 mglafl M hours C. N N

*HzSOl in excess of thatdue to CeCHSOm.

Following the oxidation of the plutonium to the hexavalent state a lay-product carrier precipitate is formed in this solution containing the oxidized plutonium. This is done by increasing the concentration of the fluoride ion, usually by adding hydrogen fluoride to make the solution 1 N in fluoride ion. A soluble alkali metal salt, such as potassium nitrate, is also added to the solution. The potassium forms an insoluble precipitate with the ceric fluoride complex, thus removing the ceric fluoride complex and excess ceric ion as the insoluble potassium ceric fluoride salt. The cerium sub-group rare earth cation of the original carrier is precipitated with the excess fluoride ion in the solution. It has been found that in order to remove the ceric ion quantitatively from the solution, it is desirable to introduce the potassium ion in at least 1:1 molecular ratio of potasium ions to ceric ion. The precipitate formed by the introduction of the hydrofluoric acid and potassium salt is quite bulky and should be washed well to insure that there is not an excessive occlusion of the hexavalent plutonium ion with the precipitate.

The plutonium is usually reduced to the tetravalent state following the by-product precipitation and separation step and rte-precipitated as the fluoride with a lanthanum or cerous fluoride carrier. By the operation of this invention the amount of carrier needed to remove the plutonium from solution is only one-fiftieth as large as the amount of the original plutonium carrier. The degree of concentration is limited mainly by the efliciency of the washing of the by-product precipitate. The loss of plutonium by this cycle is small, usually less than 2%.

The process of the present invention may be further illustrated by the following specific example.

EXAMPLE A sample of uranyl nitrate hexahydrate obtained by dissolving neutron-irradiated uranium in nitric acid was diluted so as to form a uranyl nitrate hexahydrate solution. A cerous fluoride carrier was formed in the solution by introducing a soluble cerous salt so that the concentration was mg. of cerous ion in 30 cc. of solution. The solution was then made 1.5 N in hydrofluoric acid. The precipitate thus formed contained tetravalent plutonium and was separated by centrifugation and washed first with water and then with very dilute hydrofluoric acid. The cerous fluoride product precipitate was dissolved in a solution containing 0.5 N H 50 l N HNO 0.1 M Ce; the Ca -Ge ratio in the final solution was 2:1. The solution was then digested at room temperature for one hour to effect the oxidation of the tetravalent plutonium to the hexavalent state. A byproduct precipitate was then formed in the solution by making the solution 0.3 N in potassium nitrate and 1 N in hydrofluoric acid. The final concentrations of the ceric ion, H 80 and HNO were, respective: 0.08 M, 0.4 N, and 0.8 N. The precipitate thus formed was separated from the solution by filtration and washed once with very dilute hydrofluoric acid. The supernatant solution and wash water were combined and the hexavalent plutonium contained therein was then reduced by making the solution 0.02 M in ferrous ion and digesting the solution for thirty minutes at room temperature. A product carrier precipitate of lanthanum fluoride was then formed in the solution and separated therefrom, carrying with it the tetravalent plutonium. Radiometric analysis of the precipitate showed that 97% of the original plutonium present was contained in the final precipitate. The ratio of plutonium to carrier in the final precipitate was fifty times that of the original plutonium carrier precipitate.

It will be apparent to those skilled in the art that various modifications of the present invention exist. For example, the original and final carriers may be cerium trifluoride or the original carrier may be cerium trifluoride and the final carrier may be lanthanum fluoride. In general, it may be said that any process wherein the dissolution of a cerium sub-group rare earth trifluoride plutonium carrier is effected by the formation of a ceric fluoride complex is to be considered as lying within the scope of the present invention.

What is claimed is:

1. The improvement in a process for separating plutonium from contaminants, which comprises treating a trifluoride carrier precipitate of a rare earth metal of the cerium sub-group containing plutonium in a valence state less than +5 with an aqueous acidic solution containing ceric ion, whereby said precipitate is dissolved in the solution and the plutonium oxidized to the hexavalent state, thoroughly contacting said solution with a soluble alkali metal compound and a soluble fluoride compound whereby the rare earth ions and contaminants are substantially completely precipitated as a by-product carrier precipitate, and separating the by-product carrier precipitate thus formed from the plutonium-containing solution.

2. The improvement in a process for separating plutonium from contaminants normally associated with plutonium in neutron-irradiated uranium, which comprises treating a trifluoride carrier precipitate of a rare earth metal of the cerium sub-group containing plutonium in a valance state less than +5 with an aqueous acidic solution containing ceric ion in at least equimolar ratio of ceric ion to carrier cation whereby said precipitate is dissolved in the solution and the plutonium oxidized to the hexavalent state, thoroughly contacting said solution with a soluble alkali metal compound in volume sufiicient to make the concentration of the alkali metal ion in the solution equal to the concentration of ceric ion in the solution, admixing to the solution a soluble fluoride compound whereby the rare earth ions and contaminants are substantially completely precipitated as a by-product carrier precipitate, and separating the byproduct carrier precipitate thus formed from the plutonium-containing solution.

3. The improvement in a process for separating plutonium from contaminants normally associated with plutonium in neutron-irradiated uranium, which comprises treating a lanthanum fluoride carrier precipitate containing plutonium in a valence state less than +5 with an aqueous nitric acid solution containing ceric ion in quantity greater than the lanthanum ion introduced into the solution whereby said precipitate is dissolved and the plutonium oxidized to the hexavalent state, introducing a soluble potassium compound into the solution so that the potassium ion concentration is equivalent to the ceric ion concentration in said solution, introducing hydrofluoric acid into the solution whereby a by-product carrier precipitate for fluoride-insoluble contaminants is formed,

' and separating said by-product carrier precipitate from the plutonium-containing solution.

4. The process of treating a trifluoride carrier precipitate of a rare earth metal of the cerium sub-group containing plutonium in a valence state less than +5, which comprises introducing said precipitate into an aqueous nitric acid solution between 0.5 and 3 N in HNO and containing ceric ion in quantity at least equal to the quantity of carrier cation introduced into said solution whereby said precipitate is dissolved and the plutonium is oxidized to the hexavalent state, introducing a soluble potassium compound into the solution so that the potassium ion concentration is equivalent to the ceric ion concentration in the solution, introducing hydrofluoric acid into the solution so that the molarity is between 0.2 and 1.2 M in HF, and separating the by-product carrier precipitate thus formed from the plutonium-containing solution.

5. The improvement in a process for separating plutonium from contaminants normally associated with plutonium in neutron-irradiated uranium, which comprises treating a cerous fluoride carrier precipitate containing plutonium in a valence state less than +5 with an aqueous nitric acid solution containing ceric ion in a quantity greater than the cerous ion introduced into the solution whereby said precipitate is dissolved and the plutonium is oxidized to the hexavalent state, introducing a soluble potassium compound into the solution so that the potassium ion concentration is equivalent to the ceric ion concentration in said solution, introducing hydrofluoric acid into the solution whereby a by-product carrier precipitate for fluoride-insoluble contaminants is formed, and separating said lay-product carrier precipitate from the plutonium containing solution.

References Cited in the file of this patent Harvey et al.: Journal of the Chemical Society, August 1947, pp. 1010-21.

Seaborg et al.: Journal of the American Chemical Society, vol. 70, pp. 1128-34 (194-8); report submitted March 21, 1942. 

1. THE IMPROVEMENT IN A PROCESS FOR SEPARATING PLUTONIUM FROM CONTAMINANTS, WHICH COMPRISES TREATING A TRIFLUORIDE CARRIER PRECIPITATE OF A RARE EARTH METAL OF THE CERIUM SUB-GROUP CONTAINING PLUTONIUM IN A VALENCE STATE LESS THAN +5 WITH AN AQUEOUS ACIDIC SOLUTION CONTAINING CERIC ION, WHEREBY SAID PRECIPITATE IS DISSOLVED IN THE SOLUTION AND THE PLUTONIUM OXIDIZED TO THE HEXAVALENT STATE, THOROUGHLY CONTACTING SAID SOLUTION WITH A SOLUBLE ALKALI METAL COMPOUND AND A SOLUBLE FLUORIDE COMPOUND WHEREBY THE RARE EARTH IONS AND CONTAMINANTS ARE SUBSTANTIALLY COMPLETELY PRECIPITATED AS A BY-PRODUCT CARRIER PRECIPITATE, AND SEPARATING THE BY-PRODUCT CARRIER PRECIPITATE THUS FORMED FROM THE PLUTONIUM-CONTAINING SOLUTION. 